Hard to say without seeing it, but here's a list of some of the things I've run into. First be sure you have a shoe toward the front of the truck with a shorter lining, and the longer lining is on the rear shoe. If you have two long linings on the same brake you would have had a hard time getting the drum on unless it was machined well beyond the published legal limit. Check the parking brake strut bar between the two shoes. You should be able to push it with your thumb against the anti-rattle spring pressure a minimum of 1/16". 1/8" is better and more common. If that bar is tight, also check that both shoes are returning all the way and are touching the large anchor pin on top that the return springs are hooked to. If either of those are not the case, suspect the parking brake cable is rusted tight in the partially-applied position. If this was a Ford product that cable would have been rusted tight three years ago. That is not common on a four-year-old GM product.

I saw one fellow install that strut bar upside-down on an older Grand Caravan and it rubbed intermittently on the hub. That can't be done on your truck, but be sure it is connected in the slots in the shoe frames, not above or below those slots.

Pull the shoes away from the backing plate and look at the six "lands". Those are the raised spots the shoes slide across. Over time grooves wear into those lands and the shoes can catch on them. That can't happen on most shoes because the frames have bent-over tabs that they slide on, but GM doesn't do that. The shoes just slide on the edges of the frames and since those are so thin, it's easy for them to catch in a groove. Machining the drums or installing new linings puts them in a different place on the backing plate and can put the shoe frame right in those grooves. All professionals put a light coating of high-temperature brake grease on those six lands to prevent the formation of grooves and to prevent squeaks when the shoes release.

I've seen an inexperienced mechanic attach the star wheel adjuster to the bottoms of the shoes, (the parts the linings are glued to), instead of the webbing. They couldn't even get close to installing the drum. I've heard of people mixing up the adjusters from side-to-side and they would wind down instead of up causing a low brake pedal over time.

Contaminated brake fluid is another problem that is really serious but it affects front disc brakes more so we won't discuss that. I'd start by looking at the parking brake cables. Check the lever on the shoe to be sure it's fully retracted.

Were you able to turn the drum after it was installed? If not, you may have had the shoes adjusted up too high. That can happen if the drum isn't held squarely on the hub while you're trying to turn it. Did you see any signs of gear lube leaking from the axle seal? That will contaminate the linings and can cause them to grab or chatter.